1. Field of the Invention
The present invention relates to local area networks. More specifically, the present invention relates to media access units (MAUs) used in local area networks.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Local area networks (LANs) facilitate data communication between computing systems. Currently, several types of LAN systems are used widely in the art. Most systems may be classified generally as either an Ethernet, Token Ring, or 10 Base T type local area network.
The Ethernet system is characterized by a coaxial cable which serves as a bus. The nodes of the system (e.g., terminals, printers and other devices) are connected to the cable and assigned a unique address.
A Token Ring LAN is a circular network similar to the Ethernet system with the exception that a token is used to control access to the network. The token is a packet of signals that is passed from node to node. The node that has the token has control of the network with respect to the transmission of data to and reception of data from other nodes. When the node has completed a transmission, the token is released for acquisition by another node.
The 10 Base T local area network is characterized by the use of a pair of twisted wires as a connector between nodes arranged in a star configuration around a controlling hub.
Each of these systems requires an interface between the node and the local area network (LAN). Certain systems include an internal dedicated interface and allow for the connection of an external interface as well. An Ethernet network device, for example, may have an Attachment Unit Interface (AUI) for external interfacing and an internal dedicated Medium Attachment Unit (MAU) electrically connected to the same LAN controller. Hence, there has been a need for a means by which either the MAU or AUI is mutually exclusively enabled.
Standards have been developed for certain LAN interfaces. For example, for an Ethernet like network, a standard has been set by the IEEE (Institute of Electrical and Electronic Engineers) with respect to certain physical and electrical characteristics of the MAU (see Section 8.3.2.1 of the IEEE 802.3 standard). In general, the constraints imposed by the standard dictate that the interface provide electrical isolation of the physical media from the node device circuitry.
The electrical connection made to the LAN controller by both the AUI and the dedicated internal MAU is made in the form of three or four wire pairs that are passed through an isolation network, usually a transformer, to provide isolation as dictated by the IEEE standard.
Since the LAN interface is electrically isolated from the LAN controller, a mechanism was required by which the controller could configure the LAN port such that the dedicated or internal interface was selectively enabled or disabled. Selective enablement of a dedicated or internal interface is required when an optional AUI connector port is provided, otherwise, the signal pairs from either may electrically contend if both interfaces were inadvertently used.
The conventional technique for enabling/disabling and isolating the interface from the node circuitry involved the use of a DC to DC converter. A DC to DC converter is a device that converts a DC (direct current) signal at one level to a second DC signal at a second level. This device converts power delivered to and used by the LAN controller to power that is electrically isolated and of the correct voltage and current required for operation of the interface. The converter, and hence the interface, is enabled and disabled by switching power on or off to the primary side of the DC-to-DC converter.
Unfortunately, DC to DC converters are costly, unreliable, consume much power and occupy much space on the printed circuit board of the unit and introduce undesirable radio frequency and electrical noise.
Thus, a need remains in the art for an inexpensive, reliable technique for isolating power supplied to a LAN interface in accordance with the IEEE standard which does not consume much power or space nor generate undesirable noise.